Liposomes provide several advantages in drug delivery. When administered parenterally, either by the intravenous or intramuscular route, liposomes can provide controlled "depot" release of encapsulated drug over an extended time period, and reduce the side effects of the drug, by limiting the concentration of free drug in the bloodstream. Liposomes can alter the tissue distribution of and uptake of drugs, in a therapeutically favorable way, and can increase the convenience of therapy, by allowing less frequent drug administration. Liposome drug delivery systems are reviewed in Poznansky.
The use of liposomes for drug delivery by inhalation has also been studied, as reported in co-owned U.S. patent application for "Liposome Inhalation and Method", Ser. No. 737,221, filed May 22, 1985 and now abandoned. The inhalation liposomes can be tailored, according to lipid composition, to release an entrapped drug at a selected release rate which may vary, in half life, from a few hours to several days. Further, to the extent the drug is sequestered in the liposomes, side effects related to rapid uptake into the respiratory tract and bloodstream are reduced.
The compatibility of liposomes with both lipophilic and hydrophilic drugs, and the ability to vary lipid composition to achieve a selected drug release rate are also advantageous in administering a drug topically or to mucosal tissue. An added advantage of liposome for drug delivery to mucosal tissue is that the liposome surfaces can be modified for increased tissue stickiness, to enhance the residence time of the liposomes at the target tissue site. This feature is described in co-owned patent application for "Liposomes with Enhanced Retention on Mucosal Tissues", Ser. No. 890,815, filed July 28, 1986.
Several methods for preparing liposomes with entrapped drug are known. In one method, vesicle forming lipids are deposited as a thin film on the sides of a flask, and slowly rehydrated by addition of an aqueous buffer. The drug to be entrapped may be included either in the lipid film (in the case of a lipophilic drug), or in the aqueous hydration medium (in the case of a hydrophilic drug). The liposomes that form are multilamellar vesicles (MLVs) having heterogeneous sizes between about 0.05 and 10 microns.
The MLVs may be subsequently processed, tpically by homogenization, sonication, or membrane extrusion, to produce smaller, more uniformly sized suspension. Liposome sizing down to about 0.2-0.4 microns is generally preferred. Liposomes in this size range can be sterilized by passage through a 0.45 micron depth filter, have less tendency to aggregate, and also may show more favorable organ distribution when administered intravenously (Gabizon).
One of the drawbacks of the MLV method is relatively poor encapsulation efficiency of water-soluble drugs. Typically, when the vesicles are prepared by addition of an aqueous drug solution, only about 5-15% of the total drug added to the lipid film is encapsulated in the vesicles. Liposome sizing, if needed, reduces the percentage of free drug still more, since liposome sizing methods generally result in some loss of encapsulated material.
Alternative methods for preparing liposomes with higher encapsulation efficiencies have been reported. One of these is a solvent injection, in which a lipid-in-solvent solution is injected into an aqueous medium (Deamer, Schieren, Cafiso). The method produces relatively uniform unilamellar vesicles with encapsulation efficiencies (trapping volumes) of between about 20-45 percent. The higher trapping volumes are presumably related to formation of relatively large unilamellar structures.
Increased encapsulation efficiencies can also be achieved in a reverse evaporation phase method of liposome preparation (Szoka, 1978, 1980). Here a lipid-in-solvent solution is mixed with an aqueous medium, and emulsified to form a water-in-oil emulsion. Removal of the lipid solvent produces a reverse-phase lipid gel which is then agitated, preferably in the presence of added aqueous medium, to form reverse-phase evaporation vesicles (REVs) characterized by relatively large sizes and one to a few bilayer shells. Encapsulation efficiencies for water-soluble compounds are typically between about 30-50 percent of the compound present in the original aqueous medium.
In both the solvent-injection and REV procedures, it may be necessary to reduce liposome sizes, to permit liposome sterilization by filtration and/or to improve the targeting properties of the liposomes. As with MLVs, liposome sizing leads to a loss of encapsulated material.
Since the advantages of liposome drug delivery depend on entrapment of the drug by liposomes, it is generally desirable to administer a drug in predominantly liposome entrapped form, i.e., at least about 50 percent of the drug is associated with the liposomes. This is particularly true where the drug is known to cause undesired side effects when administered in free form. The benefit of administering a water-soluble drug in predominantly liposomal form is illustrated in co-owned patent application for "Liposome Inhalation Method and System", Ser. No. 737,221, filed May 22, 1985 and now abandoned. Here it was shown that the systemic side effects of metaproteranol sulfate (MPS) were substantially reduced when the drug was delivered by inhalation in predominantly liposome-encapsulated form.
In the case of water-soluble drugs, where known liposome preparation methods yield at best 30-50% encapsulation, higher encapsulation levels (above 50% encapsulated drug) can be achieved by treating the liposomes to remove free drug. This can be done, conventionally, by molecular sieve chromatography, centrifugation, or diafiltration. In all of these methods, the bulk phase suspension medium containing the free drug is replaced by drug-free bulk medium.
One drawback of this approach is the additional processing required to remove free drug and, if desired, reclaim the removed drug. A second limitation, in the case of a water-soluble, liposome-permeable drug, is that the liposome composition must be administered before the drug can be re-equilibrate between encapsulated and bulk-phase compartments in the suspension. The second problem has been addressed in co-owned patent application for "Liposome Concentrate and Method", Ser. No. 860,528, filed May 5, 1986 and now abandoned. According to this invention, a dilute suspension of liposomes containing a water-soluble, liposome-permeable drug are concentrated to a lipid paste containing at least about 50% and preferably about 70% encapsulated aqueous volume, which also represents the percentage of drug which is encapsulated in the liposomes. The suspension is stored in concentrated form, and diluted shortly before use, i.e., the drug in the diluted suspension is administered in a non-equilibrated, predominantly encapsulated form. The removal of free drug and liposome concentration can be accomplished in a single step by ultrafiltration, centrifugation, or the like. Despite its advantages, the liposome paste approach involves loss of free drug material, and additional processing of the liposome suspension.